Visualization of surgical targets in ophthalmic surgery is the cornerstone of multiple developments ongoing in the field. Visualization is a combination of the optics of the microscopes including safety filters as well as any lenses, the incoming light source including its instruments and fibers, and finally the optical target including any visualization dyes.
Illumination sources for ophthalmic surgery can be external or integrated into vitrectomy systems like, for example, the Alcon Constellation or the Bausch & Lomb Stellaris PC. The initial light sources for vitrectomy were metal halide or halogen bulbs. The output from these light sources was typically coupled into 20, 23, or 25 gauge-probes adapted for insertion into an eye to illuminate tissue to be treated during ophthalmic surgery.
The current trend in vitrectomy and vitreoretinal surgery is for smaller gauge instrumentation. Currently 23- or 25-gauge tools are the standard but the challenge to drive this to even smaller tools (27- and 29-gauge) is greatly limited by the illumination probes (using optical fibers) that provide the required illumination. Because they are extended incoherent sources, it is difficult to couple light from metal halide and halogen bulb light sources into the smaller diameter optical fibers of these narrower gauge probes. As a compromise, light may be coupled into a larger diameter optical fiber which is then tapered to a narrower gauge probe for insertion into an eye. This tapering also leads to light loss, however. Hence, as the trend to these smaller gauge instruments progresses, the halogen light sources that were previously used lose 50% of the brightness that they had provided with 20-gauge instrumentation.
To compensate for the light loss in the narrower gauge probes a new generation of light sources was developed based on brighter mercury vapor and xenon light bulbs. There is a concern however that the spectral distribution emitted by these light sources may cause retinal damage. To address this concern filters have been added to these illumination systems to block ultraviolet and blue light. These cut off filters typically start at 420 nm, but different designers have used cut offs at 435 nm, 475 nm, 485 nm, and even 515 nm.
Use of such filters typically changes the color of the viewing environment. Ophthalmic surgeons want clean illumination so they can view what the native tissue actually looks like. Yet, the more filtering of the blue light, the more yellow-looking the tissue will be. While white light emitted by these light sources is considered to provide “better” visualization of the tissue to be treated, the tradeoff is less time to safely perform the procedure. Further, the majority of the light output from mercury vapor light sources is centered on their two natural peaks at 550 nm and 580 nm. As a result the illumination is typically green-yellow.
The visualization and reliability of current light sources have additional limitations. The life of a xenon lamp is typically approximately 400 hours but it loses power throughout its useful lifetime. Most xenon bulbs don't reach their stable output until after ten minutes of “warm up”. In addition, xenon and mercury vapor light sources are relatively inefficient and consequently heat up. As a result, cooling fans are typically required with these light sources.